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While children as young as three understand they should share, it's not until they hit seven that they practice what they preach.

Now a US study has given insights into that childhood hypocrisy in a paper published today in PLOS One.

"This is the first time that a bunch of important questions about sharing have been asked in the space of a single study," says the study's lead author Dr Craig Smith from the University of Michigan.

These questions - what a child thinks they should do, what they would do and their actual behaviour - threw up results that "were quite revealing", says Smith.

The study involved giving 102 children aged three to eight years four stickers and asking them how they should divide them with another child.

"Children as young as three were very clear that, if all were equally deserving, they and others should share half," Smith says.

Moral agents

Actual behaviour was a different story; despite knowing they should share, the three year olds kept most stickers for themselves. It wasn't until they got older that they began to divide them up fairly.

To understand that disconnect between toddlers' words and deeds, the researchers tested the children's ability to inhibit their actions and found while this did improve with age it did not explain their behaviour as the "younger children anticipated they would share less than half".

Rather than their greed being the result of a "last-minute failure of willpower", the researchers found as the children aged they attached more importance to abiding by the expected norm of sharing equally.